1. Field of the Invention
The present invention relates to ethylene polymers of high melt index and in particular to ethylene/.alpha.-olefin copolymers having a melt index of at least 100 and especially in the range 100-200.
2. Description of the Prior Art
Melt index is a measure of melt viscosity which is related to the molecular weight of the polymer. As referred to herein the melt index of a polymer is that measured by the method of ASTM D-1238 (condition E).
Ethylene homopolymers and ethylene/.alpha.-olefin copolymers, in which the .alpha.-olefin is an aliphatic hydrocarbon, are sometimes referred to as polyethylenes. Such polyethylenes are capable of being used in a wide variety of end uses, the end use depending in particular on the physical properties of the polymer. One such physical property is the melt index of the polymer. Polymers of relatively low melt index, for example, less than about 10 and especially less than about 1 and having a density of about 0.915-0.960 may be extruded in the form of film. Such film may be used as a packaging material.
Polyethylene is also capable of being fabricated into articles by moulding techniques especially injection moulding techniques. Moulding grade polyethylene is usually of higher melt index than that used in the manufacture of film. Typically, the melt index of polyethylene used in injection moulding processes is in the range of about 0.3 to 45 while the density is usually in the range of about 0.915 to 0.965. However an ethylene/.alpha.-olefin polymer containing less than 0.1% of butene-1 as the .alpha.-olefin and having a melt index of 85 and a density of 0.959 is commercially available as an injection moulding resin.
Examples of articles that may be manufactured by injection moulding processes are containers especially containers that are used for the packaging of ice cream, yogurt, margarine and the like. Such containers should be of relatively uniform thickness and should have a surface that is acceptable to the consumer. The economics of the manufacture of containers by injection moulding processes depend at least in part of the thickness of the walls of the container, especially the minimum thickness consistent with the manufacture of containers of acceptable appearance and physical properties, and the rate, e.g., cycle time, at which such containers may be manufactured. Small containers tend to be of lower wall thickness than larger containers as less flow of polymer is required during the injection moulding process in order to fill the mould. Typical wall thicknesses of containers manufactured in injection moulding processes are at least 0.7 mm. Attempts to manufacture containers of lower wall thickness e.g., less than 0.5 mm, have generally been unsuccessful especially because of process problems that result in incompletely fabricated containers, containers having non-uniform wall thickness as a result of process problems and/or containers of poor surface appearance. It is an object of the present invention to provide polyethylenes of improved processability, especially in injection moulding processes, and in particular for the manufacture of containers having a wall thickness of less than about 0.7 mm.